Add a note about deleting the work directory when upgrading the war distribution from a previous installation of JIRA

We get a lot of support cases, where customers upgrade JIRA (the war edition generally) from a previous version and are seeing bugs due to JSPs being cached from the previous version in Tomcat's work directory.

We should add a note to our upgrade docs that if customers are upgrading JIRA and installing the WAR edition in the same application server they should delete Tomcat's work directory to avoid stale JSPs from being cached.